The spectral absorption of light by visual pigments is still incompletely understood. The membrane-bound opsin and the retinylidene chromophore, each of which absorbs light only in the ultraviolet (UV),interact in a way that produce pigments with sensitivity ranging from UV to red. The best rationalization we have for describing their chemistry is that, whereas protonation-deprotonation of the Schiff base of the chromophore provides rough tuning between UV and visible wavelengths, electrostatic and other interactions between chromophore and opsin achieve fine tuning. The long-term goal is to understand how such interactions adapt retinylidene-opsin based pigments to various tasks and thus make color vision possible. Specific aims are (1) to investigate UV-absorbing visual pigment properties in the tiger salamander and test the hypothesis that a labile protonation-deprotonation of the chromophoric Schiff base is a fundamental mechanism for rough spectral tuning; (2) to test the hypothesis that UV-absorbing visual pigments have higher photosensitivity than the visible-absorbing counterparts, using carp, salamander and zebrafish as models; (3) to test the hypothesis that solvent-chromophore hydrogen bonding interactions play a role in the anion-sensitive (red) visual pigments, using zebrafish and carp as models. Collaborative research (4) is aimed at assisting others in studying; - the unusual pigment of the parietal photoreceptors of lizards; - the visual function development in the Atlantic cod; - The photoreceptor development in the zebrafish; and - the visual pigments of the quail and of a genetically- expressed rhodopsin in situ. The primary methodology is microscope-based spectrophotometry of in situ visual pigments. This technique allows accessibility to a variety of naturally-occurring visual pigments in single photoreceptor cells, genetically-engineered single cells, and small volumes of membrane suspensions or true solutions. UV and visible electronic absorption and linear dichroism spectra are recorded, and used for chemical characterization. The over-all objective is a detailed understanding of visual pigment chemistry at the molecular level.